Many disorders where attentional problems are a hallmark, such as Alzheimer's disease and schizophrenia, display abnormal regulation of the so-called default network of resting brain function that maintains internally directed thought when the mind is free to wander. These regions can be overactive or less readily deactivated with attention-demanding tasks, and excess activity is thought to impair performance. There is indication that nicotine may improve attention by aiding the deactivation of the default network, and this mechanism may be of therapeutic benefit for the above disease states. The current project aims at providing a proof of concept by demonstrating that nicotinic drugs modulate default network function. The nicotinic agonist nicotine is hypothesized to improve attention by facilitating the down-regulation of default network activity, and the nicotinic antagonist mecamylamine is hypothesized to impair attention by impeding the down-regulation of default network activity during attentional task performance.